1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and control method thereof and, more particularly, to an image forming apparatus which prevents variations of the density and tint and a control method thereof.
2. Description of the Related Art
A developing device in an image forming apparatus of an electrophotographic type or electrostatic printing type generally adopts a two-component developer mainly containing toner particles and carrier particles. Especially, most developing devices in color image forming apparatuses for forming a full-color or multi-color image use a two-component developer. The toner density (that is, the ratio of the weight of toner particles to the total weight of carrier particles and toner particles) of the two-component developer is a very important factor to stabilize the image quality.
The toner particles of the two-component developer are consumed in development and the toner density changes to decrease. Considering this, the toner density of the two-component developer is controlled constant by detecting the toner density of the two-component developer in the developing device, and controlling toner replenishment to the developing device in accordance with the detected toner density. However, the electrophotographic method and electrostatic printing method form an image using electrostatic force. If the charge amount (to be referred to as toner charge amount) in the two-component developer varies, the image density also varies.
To solve this problem, Japanese Patent Laid-Open No. 11-212343 has proposed control of estimating a change of the toner charge amount from the standstill time and humidity environment, and stirring the developer in accordance with the estimation result to stabilize the image density.
However, a predicted toner charge amount may deviate from an actual one by only feedforward prediction of the toner charge amount as in Japanese Patent Laid-Open No. 11-212343. The need arises for toner charge amount observer correction using feedback. An example of the feedback is control of creating a test pattern image (to be referred to as a patch image), detecting the density, and correcting the toner charge amount. However, the patch image density is determined not only by the toner charge amount but also by the development efficiency, so only the toner charge amount cannot be detected. It is therefore difficult to predict the toner charge amount from the patch image.